The present invention relates to ceramic dielectric compositions which have high dielectric constants (K), e.g., between about 4900 and about 5400; low dissipation factors (DF), e.g., below about 2%; high insulation resistance (R) capacitance (C) products (RC), e.g., above about 7000 ohm-farads at 25.degree. C. and above about 3000 ohm-farads at 125.degree. C.; and stable temperature coefficient (TC) characteristics in which the dielectric constant does not alter from its base value at 25.degree. C. by more than about plus or minus 15% over a temperature range from -55.degree. C. to 125.degree. C.
Multilayer ceramic capacitors (MLC's) are commonly made by casting or otherwise forming insulating layers of dielectric ceramic powder; placing thereupon conducting metal electrode layers, usually a palladium/silver alloy in the form of metallic paste; stacking the resulting elements to form the multilayer capacitor; and firing to densify the material, thus forming a multilayer ceramic capacitor. Other processes for forming MLC's are described in U.S. Pat. Nos. 3,697,950 and 3,879,645 and in U.S. patent application Ser. No. 730,711, which is incorporated herein by reference.
A high dielectric constant is important, because it allows a manufacturer to build smaller capacitors for a given capacitance. The electrical properties of many dielectric ceramic compositions may vary substantially as the temperature increases or decreases, however, and the variation of the dielectric constant and the insulation resistance with temperature and the dissipation factor, are also important factors to be considered in preparing ceramic compositions for use in multilayer capacitors.
In a desirable dielectric ceramic composition for use in a multilayer capacitor for applications requiring stability in the dielectric constant over a wide temperature range, the dielectric constant does not change from its base value at 25.degree. C. (room temperature) by more than about plus or minus 15%. The insulation resistance and capacitance product of such a composition should be more than 1000 ohm-farads at 25.degree. C. and more than 100 ohm-farads at maximum working temperature, 125.degree. C. in most cases. In addition, the dissipation factor should be as close to 0% as possible.
The method commonly used to produce such temperature stable capacitors consists of firing BaTiO.sub.3, used because of its high dielectric constant, together with minor ceramic oxide additives (dopants) which comprise minor amounts of elements or compounds which control the final dielectric properties. The degree of distribution of the ceramic oxide dopants throughout the barium titanate in the unfired state will determine such things as the extent of solid solution development during firing, grain growth, and the composition of the final fired grain and grain boundary. Thus, the efficiency of mixing is a key factor in the process to achieve the desired electrical properties in the finished multilayer ceramic capacitor. Until the present invention, however, the very minor amounts of ceramic oxide dopants have been very difficult to distribute in a homogeneous fashion throughout the blended ceramic dielectric composition.
It is well known that, in order for compositional development to take place during the firing stage of the manufacture of a multilayer ceramic capacitor, the particles of the ceramic oxide dopants of a dielectric composition must be in finely divided form to ensure adequate mixing of the ceramic oxide dopants with the BaTiO.sub.3. Ideally, in order for complete compositional development to take place during sintering of the ceramic dielectric composition, it is understood that the minor components must disperse themselves such that the environment around each barium titanate grain is the same throughout the bulk of the composition and such that the environment within each barium titanate grain is the same throughout the bulk of the composition. Typically, this is attempted by milling the components of the composition to a particle size of approximately 1 micron. Homogeneous distribution will be enhanced, however, by introducing ceramic oxide dopants of a smaller particle size, e.g., approximately 0.1 micron while continuing to use BaTiO.sub.3 particles of 1.0 micron in size. By way of illustration, using uniformly distributed powders of approximately 1 micron in spherical shape, it can be calculated that a unit of mix, prepared according to the proportions disclosed in the present invention, would contain 400 particles of barium titanate, 5 particles of niobium pentoxide and 1 particle of cobalt oxide. If, however, barium titanate powder of approximately 1.0 micron average particle size is mixed with niobium pentoxide and cobalt oxide of approximately 0.1 micron particle size, and assuming that these particles are perfectly spherical and uniformly distributed, it can be calculated that a unit of mix would contain 400 particles of barium titanate, 5000 particles of niobium pentoxide and 1000 particles of cobalt oxide. Thus for each barium titanate particle there would be approximately thirteen niobium pentoxide particles and three cobalt oxide particles. It would therefore be expected that compositional development during sintering would occur much more efficiently and the effectiveness of the ceramic oxide dopant additives would be greatly enhanced compared to that achieved by mixing 1 micron particles of the minor components.
It is well known in the art that ceramic oxide particles can be reduced in size to about 1 micron by milling techniques. It has however been impossible to mill finely divided powders on the order of 0.1 microns because milling techniques incur the risk of increasing the contamination levels of undesirable species, present in the milling media, and because milling efficiencies are significantly reduced as the particle size of the powder reaches submicron levels. The process described in this invention provides a means for enhancing uniformity of distribution of minor component dopants in a ceramic mixture before firing, and thus a means for enhancing the compositional development during sintering. This is done by precipitating minor component dopants in a finely divided form, of approximately 0.1 micron average particle size, in a controlled manner such that they are associated with major ceramic component particles. The term "associated", as used herein, identifies the heterocoagulation of unlike particles produced by precipitation in accordance with the invention as disclosed herein.
In order to precipitate 0.1 micron particles of a dopant in a slurry of 1.0 micron particles of a major ceramic component such that the 0.1 micron particles of the dopant are associated with the 1.0 micron particles of the major component, it is possible to take advantage of the surface charge properties of particles in aqueous media. These surface charge properties can be quantified in terms of zeta potential. This association maximizes the contact surface area between the two species.
It is well known that the sign and magnitude of the charge on the surface of a particle in suspension can be altered by changing the properties of the medium. Under certain conditions, it is possible to have chemically dissimilar particles in suspension which have surface charges of opposite sign. One of the most effective ways to produce particles of opposite surface charge in aqueous solution is the conventional method of altering the pH of the medium. See Reference "Dispersion of Powders in Liquids" G. D. Parfitt, Halsted Press 1969, the text of which is incorporated herein by reference.
The zeta potential of a species of particles can be determined by analysis of the behavior of the particles in suspension in a medium of a specific pH, using an electrophoresis cell in which the particle velocity is measured as a function of the applied potential gradient. The particle velocity is proportional to the zeta potential. Thus, by carrying out a series of experiments at different pH values, one will obtain a zeta potential curve, relating zeta potential and pH, which will indicate both the sign and magnitude of the surface charge of the particles in suspension over a range of pH values.
There is an important point in the zeta potential curve at which the charge on the surface of a particle is zero. This is known as the point of zero charge and sometimes is referred to as the isoelectric point (IEP). Particles in suspension at their IEP are believed to tend to agglomerate with each other due to van der Waals forces of attraction. To the contrary, particles having the same charge, either positive or negative, tend to remain separated from particles of like charge because of the coulombic forces of repulsion. If two species of particles charged oppositely to each other are in suspension, particles of the first species will attract particles of the second species and will not attract particles of the like charged species, thus forming a heterocoagulation of the species. This effect of heterocoagulation of species is important because it provides a means for associating dopant particles with major ceramic component particles and, is equally important because it prevents the homocoagulation of "like" particles. Furthermore, when the dopant particles are precipitated such that the 0.1 micron dopant particles associate in this manner with the 1.0 micron particles of the major ceramic component, then the major ceramic component particles become coated with the dopant particles. Consequently, prior to the sintering stage in the production of a multilayer ceramic capacitor, the dopant particles are precisely in the position desired to produce a uniform, dopant rich grain boundary phase surrounding a major ceramic component core grain during the sintering of the composite. This maximizes the effectiveness of the dopant as a grain growth inhibitor and enhances the electrical properties of the finished dielectric ceramic capacitor.
Thus, it is advantageous to work in a pH range in which the 0.1 micron particles of the dopant are charged oppositely to the 1.0 micron particles of the major ceramic component.
For example, where the major component is barium titanate and the dopant is niobium pentoxide, the isoelectric point of the niobium pentoxide precipitated in the process of this invention is at pH 3.1 and the isoelectric point of the barium titanate is at pH 9.0. At pH values lower than 3.1, the niobium pentoxide particles are positively charged, and at pH values higher than 3.1 the niobium pentoxide particles are negatively charged. At pH values higher than 9.0 the barium titanate particles are negatively charged and at pH values lower than 9.0 the barium titanate particles are positively charged. Therefore, in the range of pH values between 3.1 and 9.0 the niobium pentoxide particles will be negatively charged and the barium titanate particles will be positively charged. This condition favors association of the two different charged species with each other, while at the same time it causes like charged species to repel each other, and thus avoids homocoagulation which can cause uneven grain size.
The preferred pH condition would be one in which the species are oppositely charged and the magnitude of the difference between the zeta potentials of the major component particles and the dopant particles is as large as possible. This would cause the greatest attraction between the two different species, and simultaneously it would cause the greatest repulsion of like species, providing a very desirable state of dispersion of the dopant particles throughout the major component particles. For example, in the case of barium titanate and niobium pentoxide, this preferred condition would occur at pH 7, where the zeta potential of the barium titanate is +30 milivolts, and the zeta potential of the niobium pentoxide is -45 milivolts.
The advantage of precipitating the niobium pentoxide particles using the preferred pH conditions, such that the 0.1 micron particles of niobium pentoxide are associated with the 1.0 micron particles of the barium titanate and not with themselves, is that this places the niobium pentoxide particles precisely in the position desired to produce a uniform niobium pentoxide rich grain boundary phase surrounding a barium titanate core grain. This positioning of the niobium pentoxide enables it to control grain growth during the sintering of the ceramic, and the electrical properties of the dielectric ceramic capacitor are thus enhanced. It is known that niobium pentoxide diffuses very slowly at the sintering temperatures used in the production of MLC's containing barium titanate and niobium pentoxide, i.e., approximately 1300.degree. C. Consequently, if the niobium pentoxide is not distributed uniformly around the barium titanate particles in suspension during the mixing stage, then, during the sintering stage, the slow diffusion rate will lead to non-uniformly distributed niobium pentoxide in the developing microstructure, causing uneven grain growth and consequently inferior dielectric properties. Non-uniform distribution can occur when 1.0 micron particles of niobium pentoxide and 1.0 micron particles of barium titanate are mixed in a conventional manner by dry or wet mixing the ingredients in a mill jar or the like, or when the niobium pentoxide is precipitated under conditions where homocoagulation of the barium titanate or niobium pentoxide is favored.
For the purposes of the specific examples given in this invention in which niobium pentoxide and cobalt oxalate are precipitated in a suspension of barium titanate, it should be noted that it is not necessary to precipitate the cobalt oxalate such that it associates with the barium titanate. This is true because the cobalt oxide, formed from the cobalt oxalate during firing, is present as an additive to compensate for the electronic charge imbalance created by the addition of the niobium pentoxide to the barium titanates and not as a grain growth inhibitor. Cobalt oxide diffuses very quickly at 10 the sintering temperatures used to produce MLC's and, therefore, as illustrated in the examples, its effectiveness is not necessarily reduced by its being present as a 1.0 micron powder.
The process described in this invention has the advantage of producing ceramic oxide particles of the order of 0.1 microns without the problems associated with current milling techniques.
A second advantage of the process is the production of ceramic dielectric compositions with improved electrical properties, i.e., higher dielectric constants, lower dissipation factors and higher insulation resistance capacitance products than those processed by conventional mixing techniques. The higher dielectric constant achieved as a result of this process has the important advantage of allowing capacitor manufacturing companies to produce multilayer ceramic capacitors with higher capacitance values for a given chip size, or the same capacitance values at a reduced chip size, given that the number of active insulating layers and the thickness of each insulating layer are constants. The benefits are thus reduced cost and/or miniaturization.